The result of radiographic methods such as, for example, computed tomography (CT), mammography, an giography, X-ray inspection technology or comparable methods is firstly the representation of the attenuation of an X-ray beam along its path from the X-ray source to the X-ray detector in a projection image. This attenuation is caused by the transradiated materials along the beam path. Thus, the attenuation can also be understood as the line integral over the attenuation coefficients of all volume elements (voxels) along the beam path.
Particularly in the case of tomography methods, for example in the case of X-ray computer tomography, it is possible to use the reconstruction methods to calculate back from the projected attenuation data to the attenuation coefficients μ of the individual voxels. Thus, one can arrive at a substantially more sensitive examination than in the case of purely considering projection images.
Instead of the attenuation coefficient, a value normalized to the attenuation coefficient of water, the so-called CT number, is generally used to represent the attenuation distribution. This number is calculated from an attenuation coefficient μ currently being determined by measurement, and the reference attenuation coefficient μH2O, according to the following equation:
      C    =          1000      ×                                    μ            -                          μ              H20                                            μ            H2O                          ⁡                  [          HU          ]                      ,where the CT number C is expressed using the Hounsfield unit [HU]. A value of CH2O=0 HU results for water, and a value of CL=−1000 HU for air. Since both representations can be transformed into one another, or are equivalent, the generally selected term of attenuation value or attenuation coefficient denotes both the attenuation coefficient μ and the CT value in what follows.
However, it is not possible to use the attenuation value of an X-ray picture to deduce the material composition of an object, since the X-ray absorption is determined both by the effective atomic number of the material and by the material density. Materials and/or tissues of different chemical and physical composition can therefore exhibit identical attenuation values in the X-ray image.
In order to improve the informativeness of an X-ray image based on the local attenuation coefficients, it is therefore known from U.S. Pat. No. 4,247,774 A, for example, to use mutually differing X-ray spectra or X-ray quantum energies in order to produce an X-ray image. This method, used in the field of computer tomography, which is also generally denoted as two-spectra CT, utilizes the fact that materials of higher atomic number absorb low-energy X-radiation substantially more intensely than do materials of lower atomic number. By contrast, in the case of higher x-radiation energies the attenuation values become assinilated and are largely a function of the material density. By calculating the differences in the X-ray images recorded with different X-ray tube voltages, it is therefore possible to obtain additional information relating to the materials on which the individual image regions are based.
Yet more specific items of information are obtained when, in addition, the method of so-called base material decomposition is applied to X-ray images. In this method the X-ray attenuation values of an object to be examined are measured with the aid of X-ray beams of lower and higher energy, and the values obtained are compared with the corresponding reference values of two base materials such as, for example, calcium for skeletal material and water for soft part tissues. It is assumed here that each measured value can be represented as a linear superposition of the measured values of the two base materials. Thus, a skeletal component and a soft tissue component can be calculated for each element of the pictorial display of the object to be examined from the comparison with the values of the base materials, thus enabling a transformation of the original pictures into displays of the two base materials.
German patent application DE 101 43 131 A1 further discloses a method whose sensitivity and informativeness further exceeds the base material decomposition and, for example, enables a functional CT imaging of high informativeness. The method can be used to calculate the spatial distribution of the density ρ (r) and the effective atomic number Z (r) by evaluating spectrally influenced measured data of an X-ray apparatus. Body constituents such as, for example, iodine or the like can be determined quantitatively from a combined evaluation of the distribution of the density and effective atomic number and, for example, instances of calcification can be removed by segmentation on the basis of the atomic number.
However, none of the methods represented so far permit the concentration of elements and/or element combinations of which the object to be examined is composed to be determined in a spatially resolved fashion.